In general, a hot strip is manufactured by heating a slab to a predetermined temperature in a heating furnace, rolling the heated slab to a predetermined thickness by a roughing stand so as to form a rough bar, rolling the rough bar by a continuous finishing stand including a plurality of rolling stands so as to form a strip having a predetermined thickness. This hot strip is cooled by a cooling device provided on a run out table, and is then coiled by a down coiler.
In this case, in the cooling device provided on the run out table so as to continuously cool a hot-rolled strip having a high temperature, a plurality of laminar flows of coolant are linearly poured from a round type laminar flow nozzle onto strip-conveying roller tables over the width of the roller tables for the purpose of upper side cooling. On the other hand, spray nozzles are provided between the roller tables for the purpose of lower side cooling. From the spray nozzles, coolant is ejected. The above-described method is adopted normally.
In this known cooling device, however, coolant poured on the upper side of the strip then stays on the upper side of the strip after cooling, and this overcools the upper side. The overcool state is not uniform in the longitudinal direction of the strip, and therefore, the cooling stop temperature varies in this direction. Further, since the coolant from the round type laminar flow nozzle used for upper side cooling is poured in the form of free fall flows, it does not easily reach the strip if there is residual coolant on the upper side of the strip. Depending on whether there is residual coolant on the upper side of the strip, the cooling ability differs. Moreover, since the coolant falling on the strip freely spreads in the forward, rearward, rightward, and leftward directions, a cooling zone changes, and this causes thermal instability in cooling. As a result of this change in cooling ability, the material of the strip is apt to be uneven.
Accordingly, a method in which coolant (residual coolant) on the strip is purged for a stable cooling ability by obliquely ejecting fluid across the upper side of the strip so as to discharge the residual coolant (for example, see Patent Document 1) and a method in which a cooling zone is fixed by damming residual coolant with a restriction roller serving as a purging roller for restraining vertical motion of a strip (for example, see Patent Document 2) have been proposed. Further, as a cooling method for fixing a cooling zone by keeping coolant on a strip, a method for ejecting coolant from slit type nozzles inclined and opposing each other, as shown in FIGS. 11A and 11B (for example, see Patent Document 3) has been proposed.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-141322    Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-166023    Patent Document 3: Japanese Unexamined Patent Application Publication No. 59-144513